Indeed, mirc doesn't send a reply if the argument doesn't
start with a digit.
On a sidenote, your link has a typo, the ) is included in the url. Also, the ctcp draft on that page isn't
the original draft compiled by Klaus Zeuge and revised by Ben Mesander, assuming that mirc adheres to the latter. It certainly doesn't adhere to the former, as there is no irc client (including mirc) that sends a ctcp PONG reply after receiving a ctcp ping. There also other differences in it. This has nothing to do with the issue you reported, I just think you should've quoted the original draft, as it's the one with most "followers".
